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Summary: The impacts on coastal zones of sea-level rise and climate change include inundation, riverine flooding, saline intrusion, erosion, and wave damage. The impacts of changes in weather conditions (winds, waves, storms, and storm surge) may be comparable to those of sea-level rise alone. Beaches, estuaries and coastal wetlands, and reefs-including the Great Barrier Reef-have adapted naturally to past changes in climate and sea level. However, in the future they are likely to face faster rates of change, and in many cases landward migration will be blocked by human land uses and infrastructure. Impacts will be complex, both physically and socioeconomically, and will vary greatly from place to place. There is potential for considerable damage to the region's low-lying coastal settlements and infrastructure where populations, tourism, and capital investment are large and growing. New Zealand also is exposed to impacts on its Pacific island territories, including the eventual possibility of environmental refugee fluxes. Some parts of Australasia's coastline, including a number of coastal cities, and indigenous communities in low-lying coastal settlements and islands are highly vulnerable.
Adaptation options include integrated coastal zone management; redesign, rebuilding, or relocation of capital assets; protection of beaches and dunes; development zone control; and retreat plans.
The Australasian region contains diverse coastal zones spanning tropical to subantarctic latitudes and including continental coasts and small islands. The habitats of coastal zones and their adjacent small islands support a great range of ecosystems and complex food networks. A variety of economic activities are located in the coastal zone, which also provides important landscape and recreation values. Direct modification of the coastline through structural works is confined to limited areas of industrial, urban, and resort development. However, changes in sediment processes as a result of upstream land-use change and modification of areas adjacent to the shoreline and of coastal dune systems have resulted in some significant morphological changes in coastal areas. In addition, discharge of nutrients, sediments, and pollutants from agricultural and urban runoff and domestic and industrial wastes affects the composition and quality of some coastal and estuarine waters (Zann, 1995; Larcombe et al., 1996).
The Australian coastline is about 70,000 km in length (at the 0.1 km scale); it includes 12,000 islands and 783 major estuaries, 415 of which are in the tropics. About 24% of the Australian coast consists of dunes and beaches, and there are large areas of intertidal and supratidal mud, alluvium, mangroves, and seagrass, as well as thousands of kilometers of coral reefs (Zann, 1995). The New Zealand coastline is approximately 15,000 km-of which 25% is eroding, 19% accreting, and 56% (mostly hard-rock cliffs) static (Gibb, 1980); it is more tectonically active than the Australian coast.
The potential impacts of climate change on the coastal zone include the direct effects of rising sea level; rising temperatures; and possible changes in weather, storminess, and wave characteristics, together with the indirect impacts of changes on adjacent lands and rivers-resulting in altered inputs of freshwater, floodwaters, sediments, nutrients, and biota to the coastal seas. Global sea level is projected to rise 15-110 cm above current levels by the year 2100, with a central estimate of about 50 cm (IPCC 1996, WG I, Section 7.5.2). The estimated rate of rise is about two to five times faster than that experienced over the past 100 years.
Actual rates of rise at specific coastal locations can vary substantially due to local rise or fall of the land, different rates of oceanic warming and expansion between regions, and changes in oceanic and atmospheric circulation under climate change (Pittock et al., 1995; IPCC 1996, WG I, Section 7.2; IPCC 1996, WG II, Section 9.3.1). It is likely that storminess will increase in some regions and decrease in others, with parallel increases and decreases in impacts.
The coasts of Australia and New Zealand are not simple, passive systems but respond dynamically to sea level and climate in a range of ways depending on local circumstances. Weather elements and related processes such as wind speed and direction, rainfall intensities, ocean wave energies, and storm surge have major roles in coastal geomorphology with respect to the mean state and extreme events. Even small changes in weather patterns may cause impacts commensurate with those of sea-level rise alone (Cowell et al., 1996; IPCC 1996, WG II, Box 9.4; McInnes and Hubbert, 1996). Recent reviews of climate change impacts and adaptations for the region may be found in Chappell et al. (1996) and Kay et al. (1996).
Global research indicates that estuaries and coastal wetlands have coped with historical sea-level rise-for example, by migration landward. Salt marshes and mangroves have survived where the rate of sedimentation approximates the rate of local sea-level rise; beaches have grown or decayed according to changes in prevailing winds and seas; and coral reefs have demonstrated the capacity to grow vertically in response to past sea-level rise (IPCC 1996, WG II, Section 9.4). However, these past rates of natural adaptation may be insufficient for higher rates of future sea-level rise; in many cases, landward migration will be blocked by human infrastructure such as causeways, flood protection levees, and urban development, leading to a reduction in the area of the wetland or mangrove.
Sea-level rise will increase the penetration of the saltwater wedge in the many tidal estuaries, leading to salinity changes and consequent effects on estuarine ecosystems (see Section 4.3.2.1). Some coastal aquifers also may be affected by saline intrusion (Ghassemi et al., 1991).
Natural responses may be complex. For example, in macrotidal estuaries in northern Australia, channel widening from sea-level rise may contribute sediment to adjacent estuarine plains, enhancing vertical accretion. Backwater swamps and freshwater ecosystems on the estuarine plains may be endangered (IPCC 1996, WG II, Section 9.4.2). In these sediment-rich areas, where strong tidal currents redistribute sediments, mangrove communities have a better chance of survival (IPCC 1996, WG II, Section 9.4.4; see also Section 4.3.4.3 below). However, losses of coastal wetlands for Australia are expected to be greater than the global average (IPCC 1996, WG II, Section 9.5.2.3).
The potential impacts of climate change on Australia's coral reefs, including the Great Barrier Reef, have been discussed in Section 4.3.1.5. Coral reefs face the types of coastal impacts described above, questions about whether reef growth can keep pace with sea-level rise, and the problems of coral bleaching and algal invasions. For small coral islands, the higher sea level will erode inhabited coral cays and contaminate water tables to an extent that is likely to be prohibitively expensive to combat or rectify (Wilkinson and Buddemeier, 1994). On many of Australia's coral reef islands, the impacts are likely to be highly visible and would have significant consequences for tourism activities.
Climate change impacts on the coasts will be compounded by rapid growth in environmental stresses arising from existing population growth and increasing tourism.
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